1. Field of the Invention
The present invention relates to a friction brake comprising a brake part, preferably a brake caliper, on which a pressure device is arranged for pressing a friction lining against a friction surface, preferably a brake disk.
2. Discussion of Background Information
For a useful design of an electrically actuated brake such as, e.g. a disk brake of a vehicle, the actual forces and energies for actuation and wear adjustment are of interest. The pure wear adjustment takes place slowly over the life of the brake lining and serves for maintaining a certain clearance between brake disk and brake lining (normally less than one mm) during progressing wear of the brake linings (friction linings) or for ensuring that said the clearance does not become too large. When braking, the clearance has to be overcome, what has to take place in less than one second because otherwise the response time of the brake becomes too long with a safety risk. A rough assumption is that the clearance has to be overcome in approximately 1/10 sec. The forces required for overcoming the clearance by pressing are roughly in the range of several 10 N. These forces vary widely with condition (dirt, corrosion, age, etc.), size, and design of the brake. In case of an assumed clearance of 1 mm and a force of 100 N, a work of 0.1 Ws is needed, requiring 1 W if done in 1/10 sec. For the high force to press the brake lining against the brake disk in the case of full braking, several 10 kN are to be expected. The required travel for pressing arises from the elastic behavior in brake linings, brake caliper, and other highly loaded parts of the brake and is usually in the range of approximately several 1/10 mm. In case of full braking with e.g. a maximum force of 40 kN (assumed an average force of 20 kN) and 0.5 mm, thus, 10 Ws work would be needed for pressing on the brake linings, and when actuating the brake within ½ second, thus, on average, 20 W would be needed for full braking of a wheel. The necessary actuating energy and power of vehicle brakes are thus low, in particular in case of normal braking instead of full braking; however, the forces required for this are very high. It is therefore obvious to use an actuating drive with low power and to use a low-friction mechanism for generating the required high forces. Such a mechanism can be found, e.g., in DE 37 16 202 A1 in which a brake lining is arranged on a holding part, the brake lining is pressed or released by a driven cam. Between the holding part and the cam, an intermediate part is arranged on which the cam presses and which is guided in a guide. With this intermediate part, the friction forces occurring due to friction are absorbed and consequently, the brake lining is prevented from rotating together with the brake disk. Thus, self-energization is impossible in case of this brake, and it is always necessary that the pressure mechanism applies the full pressure force. Moreover, such a brake needs a large installation space so that this brake can be used only to a limited extent. Thus, such a brake is primarily used as truck brake where the required space is available.
Such pressure mechanisms for disk brakes are therefore known from truck air brakes. However, the compressed air has only roughly 1/10 of the pressure of a hydraulic brake as, e.g., for passenger cars. Because in case of trucks, the piston surface area and the diameter of the brake disk are larger, the normal force of the brake lining can only be achieved with an additional mechanical transmission which is the reason that is necessary that pneumatic brakes can be used on trucks. In case of conventional hydraulic brakes, the same pressure is applied everywhere and thus the same normal force. Because no self-energization takes place, friction force and braking torque are the same everywhere. Therefore, such hydraulic brakes provide excellent conditions for uniform braking which is the reason why in today's vehicles hydraulic brakes are still installed. Actually, the pneumatic brake is an analogy to the hydraulic brake; however, the brake lining is not directly driven with compressed air (insufficient pressure) as with hydraulics, but with the transmission cam.
The high forces required for braking cannot be readily generated by a simple lever with the lever ratio because, due to the limited forces of the actuating drive, the transmission ratios would become disadvantageous such that, mainly due to the limited space in the region of the wheels of a vehicle, it would not be possible to constructionally implement such a lever. Here, in particular the arrangement of the needed bearings for such a lever would create serious problems so that in practice, a simple lever cannot be used. An example for this is DE 103 24 424 A1, which shows a brake which, for braking, presses a support lever against a brake disk. However, the possible force transmission is limited by the installation space available for the brake and the bearings necessary to support the lever. Thus, a powerful drive would be necessary which, however, is disadvantageous in practice for the use in a vehicle, or, as in DE 103 24 424 A1, the brake has to provide a significant portion of the braking forces by self-energization.
However, highly self-energizing brakes always involve the risk of unequal self-energization and non-releasable blocking of the brake, especially if the resulting friction is higher than expected. Firstly, the higher the self-energization of a brake, the stronger is the dependency of the braking torque on the coefficient of friction and, secondly, the high self-energization is always bound to an angle which is determined by the friction coefficient. Thus, the highly self-energizing brake is very sensitive to the exact angle setting which makes controlling such a brake difficult and complicated. Thus, self-energization is always also a problem for the determination of the actual braking torque. In known brakes such as, e.g. in DE 103 24 424 A1 or DE 101 56 348 C1, which shows an electrically actuated brake, a correspondingly high effort is therefore required to prevent blocking or to be able to adjust the degree of self-energization. Thus, in DE 101 56 348 C1, e.g., two drives working against each other are provided for controlling the self-energization, whereby the complexity of controlling is increased accordingly.
From DE 10 2007 017 246 A1, a brake comprising a rotatably mounted force transmission element having ramps is implied, which is in engagement with a lining carrier. By pivoting the force transmission element by an electromotive actuator, the force transmission element is pivoted about its rotational axis, whereby the ramp, in the form of a cam control, displaces the lining carrier, on the one hand, in the rotational direction of the brake disk and, on the other, presses the lining carrier with brake lining against the brake disk. Due to the engagement between force transmission element and lining, also, an enforced self-energization occurs including the above-mentioned associated problems.